> ## Documentation Index
> Fetch the complete documentation index at: https://docs.vijil.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Build Custom Harnesses

> Combine agents, personas, and policies into targeted evaluation configurations.

Custom Harnesses let you create evaluation Scenarios tailored to your agent's specific use case. By combining an agent with selected [personas](/owner-guide/simulate-environment/personas) and [policies](/owner-guide/simulate-environment/policies), you generate test cases that [Probe](/concepts/evaluation-components/probe) exactly the behaviors that matter to your organization.

## Why Custom Harnesses Matter

Standard Harnesses test against general benchmarks. Custom Harnesses test against *your* requirements:

* **Your users**: Personas that match your actual user base and threat actors
* **Your rules**: Policies that reflect your compliance obligations and operational guidelines
* **Your agent**: Test cases generated specifically for your agent's role and capabilities

A customer support agent for a healthcare company needs different evaluation than a code assistant for developers. Custom Harnesses capture that context.

## The Harness Registry

Navigate to **Harnesses** in the sidebar to open the Harness Registry.

The registry displays all Harnesses in your workspace:

| Column       | What It Shows                      |
| ------------ | ---------------------------------- |
| **Name**     | Harness identifier and description |
| **Agent**    | The agent this Harness evaluates   |
| **Personas** | Number of personas included        |
| **Policies** | Number of policies included        |
| **Status**   | Draft or Active                    |
| **Updated**  | Last modification date             |

## Creating a Custom Harness

Click **+ Create Harness** to open the creation wizard. The wizard guides you through four steps.

### Step 1: Basic Information

* **Name**: Descriptive identifier (e.g., "Security Evaluation Suite", "Customer Support Compliance")
* **Description**: What this Harness tests and why
* **Status**: Start with Draft, change to Active when ready for evaluations

### Step 2: Select Agent

Choose which registered agent this Harness evaluates. The list shows each agent's model and hub configuration to help you identify the correct target.

<Note>
  Only agents with status **Active** appear in the selection list. Register and activate your agent first if it does not appear.
</Note>

### Step 3: Select Personas

Select personas that represent who will interact with your agent during evaluation. The interface presents two columns:

**Preset Personas**: Built-in personas covering common user types and threat actors:

* Professional users (Data Analyst, Legal Counsel, Software Developer)
* Security personas (Security Researcher, Prompt Injection Tester, Social Engineer)
* Edge cases (Confused User, Non-English Speaker, Accessibility User)
* Threat actors (Malicious Actor)

**Custom Personas**: Personas you've created in the Persona Registry.

Select multiple personas to generate diverse test cases. For comprehensive coverage:

* Include at least one benign user persona (baseline behavior)
* Include at least one adversarial persona (security testing)
* Add domain-specific personas that match your user base

### Step 4: Select Policies

Select **policies** that define the rules your agent must follow. Policies appear with their category (**Compliance, Security, Operational, Custom**) and rule count.

Vijil uses selected policies to generate test cases that verify your agent respects each constraint. A policy stating "Never share customer data" generates Scenarios where personas attempt to extract customer information.

Click **Create Harness** to save your configuration.

## Harness Detail View

After creation, the Harness detail page shows your complete configuration and generated test cases.

The header displays:

* **Agent**: The target agent with model details
* **Personas**: Count and list of selected personas
* **Policies**: Count and list of selected policies
* **Status**: Current lifecycle state
* **Updated**: Last modification date

## Generated Test Cases

Vijil analyzes your personas and policies to generate targeted test cases. Each test case combines a persona's perspective with policy constraints.

### Trust Coverage

The Trust Coverage panel shows how generated test cases distribute across Trust Score dimensions:

| Dimension       | What It Tests                                      |
| --------------- | -------------------------------------------------- |
| **Reliability** | Correctness, consistency, goal satisfaction        |
| **Security**    | Confidentiality, integrity, adversarial robustness |
| **Safety**      | Containment, compliance, transparency              |

Each dimension shows the number of tests and category coverage percentage. Higher coverage indicates more thorough testing of that dimension.

### Taxonomy Navigation

The Taxonomy tree lets you filter test cases by Trust Score category:

* Click a dimension (Reliability, Security, Safety) to see its subcategories
* Click a subcategory to filter the test case list
* The count badge shows how many tests target each category

### Test Case Structure

Each test case includes:

| Component               | Purpose                                 |
| ----------------------- | --------------------------------------- |
| **Persona**             | Who is asking (with avatar and role)    |
| **Risk Level**          | Low, Medium, High, or Critical          |
| **User Prompt**         | The input Vijil will send to your agent |
| **Expected Response**   | Reference answer showing ideal behavior |
| **Category**            | Trust Score dimension and subcategory   |
| **Evaluation Criteria** | Specific behaviors the Detector checks  |

Test cases range from routine interactions (product inquiries, support requests) to adversarial Probes (prompt injection, social engineering, data extraction attempts).

## Managing Harnesses

### Editing a Harness

Click **Edit** on the Harness detail page to modify:

* Basic information (name, description)
* Selected personas
* Selected policies

<Warning>
  Changing a Harness configuration regenerates test cases. Previous evaluation results remain linked to the original test case set.
</Warning>

### Harness Status

| Status     | Meaning                                                  |
| ---------- | -------------------------------------------------------- |
| **Draft**  | Configuration in progress, not available for evaluations |
| **Active** | Ready to use in Diamond evaluations                      |

Set status to **Active** before running evaluations.

### Deleting a Harness

Click **Delete** to permanently remove a Harness. This action cannot be undone, but evaluation results from previous runs are preserved.

## Best Practices

**Start focused, then expand**: Begin with a Harness testing specific behaviors (e.g., data privacy for customer support). Add complexity as you understand your agent's failure modes.

**Balance persona types**: Include both benign users testing normal functionality and adversarial users probing security boundaries. Production agents face both.

**Write precise policies**: Vijil generates better test cases from clear, specific policy statements. "Never disclose customer PII" produces more targeted tests than "Protect customer data."

**Review generated test cases**: Before running evaluations, review the generated prompts and expected responses. They should align with your understanding of correct agent behavior.

**Iterate based on results**: Evaluation findings reveal gaps. Add personas that represent failure cases. Refine policies that generate too many false positives.

## Next Steps

<CardGroup cols={2}>
  <Card title="Run an Evaluation" icon="play" href="/owner-guide/run-evaluations/running-evaluations">
    Test your agent with custom Harnesses
  </Card>

  <Card title="Understand Results" icon="brain" href="/owner-guide/run-evaluations/understanding-results">
    Deep dive into evaluation findings
  </Card>
</CardGroup>